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Idle Surging and flat (no power) >2000 RPM

jbaier

NAXJA Forum User
Location
Georgia
HELP! I've got (well my son does) a '98 XJ Sport, 4.0L, 5-speed, 115K miles. It's been at a couple of dealer's for the past 5 weeks (we've owned it 9 weeks). Here's the story.
We bought the Jeep and drove it for 4 weeks (ran great). Day 28, going up hill, the Jeep refused to shift into gear. Thinking we were pretty smart to insist on a 30 day warranty we take into the original dealer (VW). They claim to have a certified Chrsyler mechanic on staff.

They evaluate and determine the transmission needs replaced. They replace it would a salvage yard pull. While it is apart I have them replace the Flywheel, Clutch Assy, & Throwout bearing with parts I provided.

They complete reassembly and determine the vehicle is surging badly. They replace the distributor with an OEM unit due to original having a lot of slop. Vehicle is still surging. Dealer 1 throws hands in air and says take it to a Jeep dealer.

I drive it home, approximately 45 miles. After starting, the Jeep RPM will continuously cycle between 600 and a 1000 RPM. When you attempt to accelerate, it will ping badly greater than 1500 RPM and then goes flat at 2000 RPM (barely revs). Temperature has no effect, it does it hot or cold.

While at home over the weekend, I do some reading and end up replacing the CPS, IAC, & TPS with Autostore parts. No change. Check fuel pressure > rock steady 49#. Check compression > all cylinders within 10#. Some water coming from exhaust? Cleaned every electrical connection and ground.

Took to Jeep Dealer, they call back and say the PCM has internal failure and replace it.

After replacement, they call and say Jeep runs better, but is still not right and are performing further troubleshooting.
My questions are: The jeep ran fine until they replaced the trans, any connections? The whole issue of replacing the distributor, could it be incorrectly indexed? But they said it was running poorly before that. Where can I (or dealer) go from here?

Thanks,
Jeff
 
Does a '98 even have a distributor, or does it have the coil packs like my 2000. If it's like mine (with the triple coils built in to one rail, the distributor-like device is the Cam Position Sensor, which can be indexed. Check out the other thread running about the 2000 that stalls.
 
It has a distributor w/one coil. No coil packs. Cam Position sensor is in the distributor (it's a plate just below the rotor).

Thanks,
Jeff
 
That is a fear; I was not able to directly compare the new one to the old (the dealer did not have the ability to resurface) at the time. I was HOPING they did. But, it's the same part I used when I replaced the clutch on my '95 Jeep and had no problems. And if I remember right, it only fits on one way due to an asymmetrical bolt pattern.

Thanks,
Jeff
 
Jeff,

Is there a check engine light and even if there isn't, have you tried pulling codes to see if there isn't a stored code that can help you solve this tricky one??

Vacuum testing may also be something to consider....
 
Last edited:
Birch,
No check engine light at any time. According to dealer 2, the computer fault was not allowing it to store any codes or at least display them. I did the whole starting fluid on vac lines and manifold joints.

Red,
Almost, except it does it from the moment of starting. And it always does it, hot or cold. Turning off and restarting does not correct the problem. The article was a real eye opener. Does it still apply to 4.0s with a distributor?

Thanks,
Jeff
 
With distributor, it's a different issue, but it still needs to be close enough for the rotor to line up with the cap post for that plug. Pop cap off and see if rotor lines up to #1 post at #1 TDC compression. Are the wires on in the right order?
 
Winter,
I don't have the Jeep right now, still at dealer 2. We've verified the wires are correct several times.

Thanks,
Jeff
 
UPDATE * Re: Idle Surging and flat (no power) >2000 RPM * UPDATE

I just got back from the dealer. Everything electrical is working correctly now with the new computer. Computer is recording data and everything.

But the Jeep is not running right. The dealer feels there is a valvetrain issue. Either a weak spring, failed lifter, burnt valve, bent pushrod, worn cam, etc. Due to cost of refurb head and labor, they recommend replacing engine with a salvage pull. This seems crazy to me! Or is it me? I told them to put it back together and I'll pick it up in the morning.

I plan to pull valve cover, replace valve springs, and check the pushrods for straightness. What am I missing? Should I just replace the engine with a refurb? I've got less than 2 weeks to get this thing running for a 1100 mile drive to Texas.

Your assistance is much appreciated.

Thanks,
Jeff
 
are you sure the feul pressure isnt too high? i realize its kind of stupid to say, but you said the pressure at the rail was 49psi? thats ten pound more pressure than specified for an inline6 with mpi. pressure should be around 31psi with the vacuum tube hooked to the regulator and around 39psi with the vacuum tube detached...maybe the regulators bad?
 
No power above 2k is indicative of a distributor indexing problem. Surging is normally caused by a bad/dirty TPS or a vacuum leak. Do a search on distributor indexing. Its not hard.

If you have one handy, I would use a pressure gauge and verify pressure at the fuel rail is in specs.
 
UPDATE * Re: Idle Surging and flat (no power) >2000 RPM * UPDATE

All,
I just picked it up in frustration. As I said before everything electronic is working. The spec I have for fuel pressure was 49.2 +-5 psi. It is still surging, but it is now more uneven. The vehicle seems more responsive now though and pinging/rattling is not as severe.

The mechanic checked the distributor indexing and base timing. It was only 1 degree retarded. Well within spec and capability of PCM to manage.
Their final diagnosis (to tune of $783, with PCM replacement) is a valvetrain issue indicated by >40% cylinder leakdown on Cylinders #2 & #4, a good vacuum but rapidly oscillating (needle jumping up and down on gauge within a tight range). This is resulting in exhaust gas leaking back into the intake causing the surging/bucking and loss of power. Dealer recommends Salvage pull engine replacement at a price of approx. $3500 w/thier labor.

What do you all think?

My wife has a theory to what happened. My son, being a relatively new and young driver, missed his shift when going up a large hill. Forcing the trans into 2nd gear at approx. 60 mph and dumping the clutch. This significantly oversped the engine, resulting in the trans damage and damage to the valve train.

What do you all think?

Where should I go from here? I'm leaning to just putting in a refurb engine myself, based on 115K already on the engine.

Thanks,
Jeff
 
89xj,
I thought about, but I just don't have the time. Just for the valve springs, the autoparts said 5 to 10 days; that does not even include any possible machining required.

Then, I thought refurb head. With bolts, gaskets, 1 yr warranty, etc. it comes to about $600. I could have by this weekend. But, if I find more problems that delays me more. And, I still have a new top end on a 115K bottom end.

JY vs Refurb engine. This engine is going into my son's Jeep, he's going to live 1100 miles away (at college). Locally a low mile (<50K) JY engine is about $900 w/30 day warranty. For $1600 I can get a refurb w/3 year nationwide warranty. I can have the engine by this weekend.

My real question in all this is; does the dealer's diagnosis seem correct? Is my wife's theory sound possible?

Thanks,
Jeff
 
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